Solid electrolytes have been shown to have many advantages in the fabrication of electrochemical cells and batteries, such as thermostability, reduced corrosion of the electrodes, and cyclability. Furthermore, solid electrolytes permit us to create electrochemical sources of high energy per unit weight. Solid electrolytes, particularly polymeric electrolytes, have the principal advantage of being prepared in thin layers which reduces cell resistance and allows large drains at low current densities.
In the design of solid polymeric electrolytes both the properties of ionic conductivity and mechanical strength must be provided. It has been found advantageous to incorporate inorganic ion salts and solvents into the solid electrolytes, as well as to select polymers which enhance ionic conductivity. Cross-linking of the polymers can lead to stronger solid electrolytes, i.e. resilient thin layers of electrolyte, but cross-linking must not be to the detriment of ionic conductivity. Thermal and radiation-induced cross-linking (curing) have been extensively used for this purpose. Prior to crosslinking, the polymer or oligomer is termed a prepolymer or polymer precursor. U.S. Pat. No. 4,654,279 describes a two-phase solid polymeric electrolyte consisting of an interpenetrating network of a mechanically supporting phase consisting of cross-linked polymers, and a separate ionic conducting phase consisting of a metal salt and a complexing liquid therefore, which is a poly(alkylene oxide).
Poly (alkylene oxide), optionally derivatized with acryloyl and urethane groups is a polymer precursor for single-phase polymeric electrolytes. U.S. Pat. No. 4,908,283 discloses an acryloyl-derivatized solid polymeric electrolyte. However, radiation-cured solid polymeric electrolytes may lack sufficient mechanical strength and toughness. It is believed that the physical robustness of the cross-linked polymer is diminished by the presence of high molecular weight poly(oxyalkylene) units in the polymer precursors. Such poly(oxyalkylene) units are referred to as the "soft sectors" of the cross-linked polymer precursors because of this physical property. The art is seeking means for strengthening the poly(oxyalkylene) portions of the polymer precursor.
The chemical cross-linking of poly(alkylene oxide), for example, as disclosed in U.S. Pat. No. 3,734,876, was suggested as an alternative to radiation-induced cross-linking in order to obtain more control over the product's properties and synthesis.
The use of di-, tri- and polyisocyanates to create urethane linking groups between poly(oxyethylene) units for use as solid polymeric electrolytes is reported in U.S. Pat. No. 4,357,401. However, Fiona M. Gray, "Solid Polymer Electrolytes", VCH Publishers, Inc., New York, 1991, at page 103, reports that the incorporation of isocyanate units into the network polymer results in a polymeric electrolyte containing large quantities of bulky groups which are superfluous to the conduction mechanism and hinder ionic motion. In addition, it is reported that the urethane linkage has a strong influence in the glass transition temperature because of interactions between the urethane linkage and the poly(oxyalkylene) units.
It would be advantageous if thermal and radiation-curable polymer precursors based on urethane linked poly(oxyalkylene)glycols could be designed to lend mechanical strength to the solid polymeric electrolyte without loss of ionic conductivity.